Use of magnetically-stiffened magnetorheological fluids (MRF) for abrasive finishing and polishing of substrates is well known. Such fluids, containing magnetically-soft abrasive particles dispersed in a liquid carrier, exhibit magnetically-induced plastic behavior in the presence of a magnetic field. The apparent viscosity of the MRF can be magnetically increased by many orders of magnitude, such that the consistency of the MRF changes from being nearly watery to being a very stiff paste. When such a paste is directed appropriately against a substrate surface to be shaped or polished, for example, an optical element, a very high level of finishing quality, accuracy, and control can be achieved.
U.S. Pat. No. 5,951,369, issued Sep. 14, 1999 to Kordonski et al., discloses methods, fluids, and apparatus for deterministic magnetorheological finishing of substrates. This patent is referred to herein as “'369.”
In a typical magnetorheological finishing system such as is disclosed in the '369 patent, a work surface comprises a vertically-oriented non-magnetic wheel having an axially-extending rim which is undercut symmetrically about a hub. Specially-shaped magnetic pole pieces are extended toward opposite sides of the wheel under the undercut rim to provide a magnetic work zone on the surface of the wheel, preferably at about the top-dead-center position. The surface of the wheel is preferably an equatorial section of a sphere.
Mounted above the work zone is a substrate receiver, such as a rotatable chuck, for extending into the work zone a substrate to be finished. The chuck is programmably manipulable in a plurality of modes of motion and is preferably controlled by a programmable controller or a computer.
MRF is extruded in a non-magnetized state from a shaping nozzle as a ribbon onto the work surface of the rotating wheel, which carries the fluid into the work zone where it becomes magnetized to a pasty consistency. In the work zone, the pasty MRF does abrasive work, known as magnetorheological polishing or finishing, on the substrate. Exiting the work zone, the fluid on the wheel becomes non-magnetized again and is scraped by a scraper from the wheel work surface for recirculation and reuse.
Fluid delivery to, and recovery from, the wheel is managed by a closed fluid delivery system such as is disclosed in the '369 reference. MRF is withdrawn from the scraper by a suction pump and sent to a tank where its temperature is measured and adjusted to aim. Recirculation from the tank to the nozzle, and hence through the work zone, at a specified flow rate may be accomplished, for example, by setting the speed of rotation of a pressurizing pump, typically a peristaltic or centrifugal pump. Because a peristaltic pump exhibits a pulsating flow, in such use a pulsation dampener is required downstream of the pump.
The rate of flow of MRF supplied to the work zone is highly controlled. An inline flowmeter is provided in the fluid recirculation system and is connected via a controller to regulate the pump.
A capillary viscometer is disposed in the fluid delivery system at the exit thereof onto the wheel surface. Output signals from the flowmeter and the viscometer are inputted to an algorithm in a computer which calculates the apparent viscosity of MRF being delivered to the wheel and controls the rate of replenishment of carrier fluid to the recirculating MRF (which loses carrier fluid by evaporation during use) in a mixing chamber ahead of the viscometer, to adjust the apparent viscosity to aim.
U.S. Pat. No. 5,616,066, issued Apr. 1, 1997 to Jacobs et al. ('066), discloses a magnetorheological finishing system comprising a permanent ring magnet having north and south soil iron ring pole pieces fixedly disposed on a non-magnetic mount within a non-magnetic drum which provides a carrier surface on its outer surface.
A serious shortcoming of the '066 system is the inability to finish concave surfaces because of the cylindrical carrier wheel surface.
A further shortcoming is that a permanent magnet provides only one value of magnetic field, and thus control of removal rate by varying the strength of the magnetic field is not possible.
A still further shortcoming is that a permanent magnetic field makes difficult the cleaning and maintaining of the system for the fluid changeover.
U.S. Pat. No. 6,506,102, issued Oct. 30, 2001 to Kordonski at al. ('102), which is hereby incorporated by reference, improves upon the '066 system and discloses a system for magnetorheological finishing which comprises a vertically oriented carrier wheel having a horizontal axis. The carrier wheel is preferably an equatorial section of a sphere, such that the carrier surface is spherical. The wheel is generally bowl-shaped, comprising a circular plate connected to rotary drive means and supporting the spherical surface which extends laterally from the plate. An electromagnet having planar north and south pole pieces is disposed within the wheel, within the envelope of the sphere, and preferably within the envelope of the spherical section comprising the wheel. The magnets extend over a central wheel angle of about 120° such that MRF is maintained in a partially stiffened state well ahead of and well beyond the work zone. A magnetic scraper removes the MRF from the wheel as the stiffening is relaxed and returns it to a conventional fluid delivery system for conditioning and re-extrusion onto the wheel. The placement of the magnets within the wheel provides unencumbered space on either side of the carrier surface such that large concave substrates, which must extend beyond the edges of the wheel surface during finishing, may be accommodated. The angular extent of the magnets causes the MRF to be retained on the wheel over an extended central angle thereof, permitting orientation and finishing in a work zone at or near the bottom dead center position of the wheel.
A benefit of the '102 system is that use of an electromagnet rather than a permanent magnet enables another control parameter, i.e., the intensity of the magnetic field, to be varied by varying the current amperage supplied to the electromagnet.
A shortcoming of the '102 system is that the increased size of an electromagnet (in comparison to an equivalent-strength permanent magnet) imposes limitations on the minimum size of the spherical wheel, and thus limits the smallest radius of curvature of concave substrates to be finished.
What is needed in the art is an MRF system having a smaller-radius spherical finishing wheel.
It is a principal object of the present invention to finish smaller-radius concavities than is heretofore possible using prior art MRF systems.
It is a further object of the invention to provide a system for magnetorheological finishing of concave substrates wherein the radius of the work piece concavity is not limited by the size of magnetic system.
It is a still further object of the invention to provide a system employing permanent magnets for magnetorheological finishing of substrates wherein the finishing may be carried out at any desired magnetic field strength.
It is a still further object of the invention to reduce maintenance cost and electrical power consumption in magnetorheological finishing.